Electronic circuit breakers have electronic components inside them that require a power supply, which includes a transformer followed by a bridge rectifier that converts the alternating current to a corresponding direct current. When turned on and current is flowing through the circuit breaker, it provides thermal protection as it monitors the current passing through the circuit breaker to a load being protected by the circuit breaker. Circuit breakers have long provided thermal protection to protect the insulation on the primary conductors. If the primary conductors become too hot, their insulation will melt and its insulating properties will be compromised. A traditional mechanical circuit breaker provides this protection by passing current through a bimetal, which deflects as a function of temperature. When current passes through the bimetal, its heating models that of the primary conductors, which are also carrying current. As typically applied, electronic circuit breakers provide protection by measuring the potential created in a burden resistor when secondary current passes through that resistor. However, the instant the primary current stops flowing, the secondary current drops to zero, but the primary conductors remain at an elevated temperature. If primary current starts to flow again within approximately 15 minutes, significant residual heat remains in the primary conductors, so they are still at an elevated temperature. The mechanical bimetal remains partially deflected until it is totally cooled. If the circuit breaker is powered up when residual heat remains in the primary conductors, the circuit breaker lacks any “memory” of the thermal history of the primary conductors and may not recognize quickly enough that a thermal fault still persists, compromising the integrity of the insulation on the primary conductors.
One approach to modeling thermal memory uses a timer, which assumes that the primary conductors cool at a fixed rate over a given period of time. The timer adds circuit complexity and cannot account for variations in the heating or cooling of the primary conductors such as caused by ambient temperature conditions.
Another approach uses a resistor-capacitor (RC) circuit that holds a voltage proportional to the temperature of the system. The RC circuit adds cost and complexity to the system.